Plugging something into the headphones jack on a guitar amp usually kills the speaker, but not always. I actually have an old Marshall combo that has a headphones/line out jack. Plugging something into it does not effect the speaker. Some amps have an actual line out, and not a headphones out, so that would be preferred. But if you're using the headphones as a line out, you'd be able to listen to your guitar through your PC speakers as you're recording.
USB interfaces are great. I have this one:
http://tascam.com/product/us-200/overview/
Comes with Cubase LE for free. Most USB devices like that will come with some free software to use. Cubase is alright, but I'm still using Audition. Here's something I did with that Tascam and Audition for example:
But using the line in or mic in on a PC's sound card is perfectly fine for quick recordings. If the line in isn't working out for ya for whatever reason, you can just use a cheap PC mic. They don't all suck as bad as you would think. There's this one that use to come with Dell PCs...
http://www.ebay.com/itm/Dell-Black-Plug ... 484f8b87db
They're actually REALLY good for what they are. They sound very decent. If you're not looking to spend any money, that's what I would do. Otherwise, the USB interfaces are awesome, and an SM57 is about $90 (or less if you buy one used).
